Offshore equipment packages are often placed on the ocean floor by lowering on a cable to the desired location. Especially when one package is to be landed on the top of another package, the surge of the vessel on the surface can cause damage to the subsea equipment being landed and the equipment being landed on as they impact one another. Often times the packages must be landed securely before hydraulic connectors can join them together.
Motion compensation is used to detect the motion of the vessel and pay the supporting line in and pull it back by operating the winch as the vessel heaves so that the package near the seafloor can remain vertically stationary and then slowly be lowered to engagement. The default material for the lifting line is a high strength steel cable.
Running heavy subsea packages to depths such as 10,000 feet have the problem that when that much steel cable is deployed, the majority of the cable's strength is consumed in simply holding its own weight as steel cable is relatively heavy.
If a near neutrally buoyant synthetic rope is utilized, most of the cable's strength is retained to lower or lift the subsea load. Synthetic rope is basically a rope made of a high strength form of plastic, so it is relatively light per unit volume.
Wire cable or synthetic rope can be handled on single drum winches or winches which are comprised of a multiplicity of powered sheaves or pulleys. In either case the wire rope or synthetic rope is flexed once in lowering, but is repeatedly flexed during motion compensation times. This is of little consequence in wire cable, but the sliding friction (internal and on the sheaves) of the synthetic rope will generate high heat, potentially to the point of destroying the synthetic rope due to melting.
The capability to lower subsea packages to the seafloor with a synthetic rope under motion compensation control would allow substantially expanded deep sea lifting and lowering capabilities as contemporary winching methods for synthetic rope has the potential for damaging the critical synthetic rope during motion compensation operations.